Tuesday, 2 August 2011

Isn't BIM just 3D CAD?

Quite often when I'm out and about I'll hear people talking about the BIM and what they really mean is the 3D CAD model.
(Update: Also see linkedin groups discussion thread: "Where is the "I" of BIM If a model is used only for clash detection and generating 2D drawings")

In the article I have written for the August NBS e-bulletin I ask the question "Isn't BIM just 3D CAD?". This article is also posted on thenbs.com/BIM

A CAD example
When analysing the benefits of Building Information Modelling it is often worth taking a step back and looking at a very simple example:

Consider an external wall (as illustrated in Figures 1 and 2). Within the latest CAD systems, walls are now three dimensional objects. The wall object is then broken down into the key products that make up its structure. For example, render, external brick leaf, cavity insulation, internal block leaf and plasterboard dry lining. Each of these is an object itself; this allows the creation of automatic schedules and quantity take-off. For example, within a click of a button, the number of bricks or the sheets of plasterboard within the building can be calculated.
Figure 1 – An 'out of the box' wall in 3D CAD

Figure 2 – Products that make up this wall's structure in 3D CAD

In addition to automatically generated quantities, 3D CAD models from different disciplines can be combined for clash detection. Users can add quite complex constraints so that the objects interact as expected. And, of course, spectacular visualisations can be created. However, can the full benefits of BIM be realised in these present 3D CAD models?

Consider the same external wall in a master specification system, for example, NBS Domestic Specification, our product for domestic new build, refurbishment and alteration work. Figure 3 displays the template description of the wall as a system in NBS Domestic Specification.
Figure 3 – External wall description in NBS Domestic Specification

Additional key products
Immediately, it is apparent that in addition to the key materials such as bricks, blocks and insulation, there are a number of other products that are not described as objects in the 3D CAD model. The external wall in NBS describes wall ties, cavity trays, weep holes, and lintels, and it's also evident that key products vary above and below the damp proof course.

Product definitions
Below this system description of the external wall, each of the products that make up the wall may be defined in greater detail, as for blocks in Figure 4.
Figure 4 – Specification clause for one of the many products that make up the wall

In the vast majority of cases this detailed information would not be in your 3D CAD model. For example, what standard a particular product must comply to or what its compressive strength and thermal conductivity is.

The question arises: "Will this information one day be in CAD?"'. If so, the follow-up question is: "Who will maintain its currency?".

A true BIM must contain all of the information required to build and maintain the building. The expected standards for workmanship are a crucial part of this. Figure 5 displays a sample of the level of detail required to specify workmanship for our external wall.
Figure 5 – Specification clause to ensure high quality workmanship

Performance requirements and their verification
The final component of a true BIM that this article considers is performance requirements - crucial in many designs. In modern procurement the designers quite often produce outline schemes and describe the included systems in terms of their performance requirements.

For instance, our external wall will not be specified in terms of the materials it is made from, but in terms of its structural, acoustic, thermal or aesthetic performance. However, listing performance requirements is only half of the picture, how these requirements are to be verified once the wall is complete is also essential. Master specification systems world-wide are increasingly providing and maintaining this content. This information must now be linked to the corresponding objects in 3D CAD systems.

It is clear that the use of 3D object-based CAD packages provide huge benefits over traditional 2D CAD. However, to really appreciate the true benefits of BIM, the information in 3D CAD models must be coordinated with information in master specification systems.

Figure 6 is an illustration from the very first edition of NBS in 1973. It shows the information on the drawing coordinated with the specification, quantities, standards, regulations and manufacturer information.
Figure 6 – Co-ordinated project information

Technology is now allowing us to accurately and more efficiently coordinate this information. This process has adopted the buzz word 'BIM'. The 3D CAD example that has been considered in this article is a simple wall, but multiply this across all of the systems and products that make up a building and its surrounding landscape and it is clear that integrating CAD and specification information is a vital step to truly adopting BIM.

When you say you have adopted BIM, pause for a moment. Ask yourself have you really adopted BIM or are you currently just using 3D CAD?


  1. It is all very interesting to have all the information linked between the model and the NBS software. My view is, would it really beneficial to all project works including smaller projects? From my basic 3d cad experience, setting up the drawing , say, wall element, takes considerably lots of time if there is no template already in place. In the design feasibility stages, all the client would care would be just simple line drawings, or maybe a simple 3d model. It is, of course important to include all the info you could get your hand into the model so it could save time when it progress further. But 9/10 times the client will change their minds and all back to the drawing board. If it's new built, I could see the use of BIM, but for extension works and refurbishment programs, how would BIM be helping the situation? Also I would like to ask for some firms, NBS is not the standard way of writing specifications. Again, how would cad programs like Revit would beneficial to these companies if they do the migration to BIM?

  2. Hi Vincent,

    I don't think BIM is just about new build. Creating an "as-built" model of a building you have and then using it for alteration and refurbishment work through the life of the building arguably provides greater benefits than using BIM for the design/construction of a new building.

    Is BIM more beneficial on larger projects? Maybe, but I have seen some very good case studies of BIM usage on smaller projects. Check out the website below for some good examples:

    Can NBS be used on smaller projects? Absolutely, check out:
    http://domestic.thenbs.com for simple domestic work and also NBS Scheduler for more smaller projects that are more complex:

    Your final point about those not using master specification systems... I'd ask how they provide their clients with a robust, accurate and up to date specification? With new building technologies emerging all of the time and with around 150 construction reference documents being updated each month this is quite a task. At NBS we have around 25 construction professionals doing this job full time.


  3. Excellent and accurate information! I have just purchased 3d Cad software for my graphic design company and I couldn't be more pleased with the results and precision we get from it. Thanks for sharing!